Explicit bounds for graph minors

Abstract

Let $Σ$ be a surface with boundary $b(Σ)$, $\mathcal{L}$ be a collection of $k$ disjoint $b(Σ)$-paths in $Σ$, and $P$ be a non-separating $b(Σ)$-path in $Σ$. We prove that there is a homeomorphism $ϕ: Σ\to Σ$ that fixes each point of $b(Σ)$ and such that $ϕ(\mathcal{L})$ meets $P$ at most $2k$ times. With this theorem, we derive explicit constants in the graph minor algorithms of Robertson and Seymour. We reprove a result concerning redundant vertices for graphs on surfaces, but with explicit bounds. That is, we prove that there exists a computable integer $t:=t(Σ,k)$ such that if $v$ is a '$t$-protected' vertex in a surface $Σ$, then $v$ is redundant with respect to any $k$-linkage.

24 pages, 0 figures